Fuel cells are a kind of power generator that extract electric energy through electrochemical oxidation of fuels such as hydrogen and methanol. In recent years, fuel cells have drawn attention as a clean energy supply source. Among fuel cells, a polymer electrolyte fuel cell is operated at a low standard working temperature of approximately 100° C., and provides high energy density, and thus is expected to be widely applied as relatively small-scale distributed power facilities and as mobile power generator on automobiles, ships and the like. In addition, the polymer electrolyte fuel cell also draws attention as power source of small-scale mobile apparatus and portable apparatus, and is expected to be mounted on cell phones, personal computers and the like, in place of secondary battery such as nickel-hydrogen battery and lithium-ion battery.
A normal fuel cell is constituted of cell units, the cell unit having a configuration of a membrane electrode assembly (hereinafter referred to also as MEA) being sandwiched between separators, which MEA is constituted of an anode electrode and a cathode electrode in which a reaction of power generation occurs, and a polymer electrolyte membrane serving as a proton conductor between the anode and the cathode. Although the main component of the polymer electrolyte membrane is an ionic group-containing polymer (polymer electrolyte material), a polymer electrolyte composition containing an additive and the like can also be used to increase durability.
The characteristics required of the polymer electrolyte membrane include, first, high proton conductivity, specifically high proton conductivity even under high temperature and low-humidification conditions. Since the polymer electrolyte membrane also functions as the barrier that prevents direct reaction between fuel and oxygen, low permeability of fuel is required. Other characteristics include chemical stability to withstand a strong oxidizing atmosphere during operation of the fuel cell, mechanical strength and physical durability of being capable of withstanding thinning of the membrane and repeated swell-drying cycles.
Conventionally, as the polymer electrolyte membranes, there is widely used Nafion (registered trademark, manufactured by DuPont) which is a perfluorosulfonic acid based polymer. Since Nafion (registered trademark) is manufactured through multistage synthesis, it has a problem of being extremely expensive and large fuel-crossover (transmission amount of fuel). In addition, as to Nafion, there have been pointed out a problem of losing membrane mechanical strength and physical durability by swelling-drying, a problem in which the use at high temperatures is not possible because of low softening point, a problem of waste disposal after use, and further an issue of difficulty in recycling the material. On the other hand, the development of hydrocarbon-based electrolyte membranes has been also actively conducted in recent years as a polymer electrolyte membrane having excellent membrane characteristics at a low price and being capable of substituting for Nafion (registered trademark).
However, those polymer electrolyte membranes have a problem of insufficient chemical stability in the use for polymer electrolyte fuel cells. Although the mechanism of chemical deterioration has not yet fully been clarified, it is believed that, as a result of break of the polymer chain and the side chain by hydrogen peroxide having strong oxidizing power produced during power generation, and hydroxy radical produced by a reaction of a very small amount of metal such as iron which may exist in the system with hydrogen peroxide, due to thinning and weakening of the polymer electrolyte membrane and increase in fuel permeation, hydrogen peroxide, hydroxy radical and the like are further produced, and membrane degradation progresses with increasing speed. In addition, during repeated swelling and shrinking in association with changes in humidity, there has been a problem in which the weakened polymer electrolyte membrane breaks and thus power generation does not become possible.
In the above situation, there have been conducting studies to improve the chemical stability and durability by using a polymer electrolyte composition applying perfluoro-based electrolyte membrane and hydrocarbon-based electrolyte membrane each containing antioxidant.
For example, Japanese Patent Laid-Open Nos. 2003-151346 and 2000-11756 propose polymer electrolyte compositions adding a phosphorous-based antioxidant. Specifically, a polymer electrolyte composition adding a phosphorous acid ester (phosphite)-based antioxidant to a sulfonic acid group-containing polyethersulfone-based polymer, and a polymer electrolyte composition adding a phosphonic acid group-containing polymer such as polyvinylphosphonic acid to a sulfonic acid group-containing polyethersulfone-based polymer or a sulfonic acid group-containing polyetherketone-based polymer are proposed.
Japanese Patent Laid-Open Nos. 2003-201403, 2007-66882 and 2005-213325 propose electrolyte compositions adding sulfur-based, amine-based, phenol-based antioxidants and the like, in addition to phosphorous-based antioxidants. Specifically, a polymer electrolyte composition adding an antioxidant such as phosphorous acid ester (phosphite), thioether, hindered amine or hindered phenol to a sulfonic acid group-containing polyethersulfone-based polymer or a sulfonic acid group-containing polyarylene-based polymer are proposed.
Japanese Patent Laid-Open No. 2006-99999 proposes a polymer electrolyte composition adding cerium ion or manganese ion to a perfluorosulfonic acid-based polymer and a sulfonic acid group-containing polyetherketone-based polymer.
WO 2013/94538 A proposes a polymer electrolyte composition adding a phosphorus-containing additive selected from phosphine compounds and phosphinite compounds, and further a transition metal atom such as cerium or manganese.
Japanese Patent Laid-Open No. 2007-38213 proposes a peroxide decomposition catalyst coordinated to a base metal atom such as manganese or iron by a nitrogen atom such as imidazole or pyridine. WO 2011/57768 A and WO 2011/57769 A propose a polymer electrolyte composition adding a phenanthroline derivative or a complex of phenanthroline and cerium ion or manganese ion to a perfluoro-based electrolyte membrane.
However, in the polymer electrolyte compositions described in JP '346, JP '756, JP '403, JP '882 and JP '325, a general antioxidant and a light stabilizer that suppresses deterioration of plastic materials due to heat and light are only added, and they cannot obtain satisfactory chemical stability and durability of polymer electrolyte compositions under the conditions like fuel cell operating environments (high temperature, humidified, strong acidity).
Also, 2,2′-bipyridyl and 1,10-phenanthroline described in WO '768 may be oxidized by hydrogen peroxide and hydroxy radical produced during operation and eluted outside of the membrane. Thus, they cannot be said to obtain satisfactory chemical stability and durability.
In addition, in JP '999, because the sulfonic acid group is ion-exchanged by cerium ion or manganese ion that is a polyvalent metal, there are problems of deterioration of proton conductivity of the polymer electrolyte composition, deterioration of solvent solubility and solution membrane-forming ability due to ion cross-linking, and embrittlement of the membrane.
Further, a phosphorous-based additive in WO '538, 2,2′-bipyridyl in JP '213, 1,10-phenanthroline in WO '769 and the like are allowed to form a coordination (complex) structure with the metal, thereby relaxing the ion cross-linking, and improving durability while maintaining solvent solubility and membrane-forming ability. However, the complex structure is comparatively hydrophilic and may be eluted outside of the membrane during operation, thus they cannot be still said to obtain satisfactory chemical stability and durability.
As described above, the prior art polymer electrolyte compositions are insufficient in economy, processability, proton conductivity, mechanical strength, chemical stability, and physical durability. Thus, they cannot serve as industrially useful polymer electrolyte compositions.
It could therefore be helpful to provide a highly practically applicable polymer electrolyte composition having excellent chemical stability of being able to be resistant to a strong oxidizing atmosphere during operation of fuel cell, and being capable of achieving excellent proton conductivity under low-humidification conditions, excellent mechanical strength and physical durability, and provide a polymer electrolyte membrane, a membrane electrode assembly, and a polymer electrolyte fuel cell each using the same.